1. Field of the Invention
This invention relates to a syringe for the controlled discharge of pasty materials, and, more particularly to a syringe having a plastic cartridge which holds the material and into which, from one end, a rotary piston is inserted.
2. Description of the Prior Art
A syringe of the prior art is described, for example, in the product information published by Heraeus Kulzer GmbH "Charisma--Inlays--Gewinn durch Perfektion und Asthetik" [Charisma Inlays--Profit from Perfection and Appearance] (31292/D125 sK dt./WPR 12 12 200). These syringes, which contain the viscous dental materials sold under the name "Charisma" (Charisma is a registered trademark of Heraeus Kulzer GmbH), have a cartridge into which the dental material is filled. This cartridge is tapered on its discharge end over its outside circumference in the form of a material discharge nozzle and is closed by attaching a cap. In the end of the cartridge opposite the discharge end, a rotary piston is inserted into a relatively rigid, sleeve-shaped stopper which is in contact with the viscous material contained in the cartridge. The rotary piston is provided with a threaded portion which is guided in a bearing in the form of a nut. To discharge the dental materials from the cartridge, the cap is removed from the cartridge and the rotary piston, which has a handle on one end, is twisted into the cartridge so that the stopper is pushed toward the discharge end of the cartridge and applies pressure to the material. The syringes described above have been used successfully for years.
The cartridges described above which hold the dental materials are generally filled from the rear end, i.e., the end which is opposite the discharge nozzle. After the cartridges are filled from the rear end, the above-mentioned sleeve-shaped stopper is pressed into the open filling end of the cartridge until it is in contact with the filled-in material. Such prior art cartridges which hold the dental material have a slight conicity on the inside, toward the end from which the dental material is filled and the stopper is inserted. This conicity is necessary for removing a workpiece after forming the cartridge by injection molding. Therefore, the stopper is in contact with different tension in the axial direction of the cartridge against the inside wall of the cartridge as it moves from one end of the cartridge to the other. There can be problems with leaks in the cartridge, especially when low-viscosity materials are used. If a stopper is used which has a tighter seal at the transition to the inside wall of the cartridge, during the process of filling the cartridge with material after insertion of the stopper, an air pocket is formed between the material and the stopper and the air pocket cannot escape. To counteract this problem, such stoppers were provided with a hole in the center. A rotary spindle was provided with a mandrel which snapped into the hole to close it. After the material has been discharged from the cartridge and the pressure on the stopper is to be relieved, if the rotary piston is twisted too far out of the syringe, the mandrel can be pulled out of the hole in the stopper thus exposing the material in the cartridge to the atmosphere. This can result in a hardening of the material or a change in the characteristics of the material.
An essential requirement of such syringes is that it must be possible to discharge a precisely controlled amount of the material which is used for the spot to be treated or worked on a tooth or on a dental prosthesis. Moreover, such a syringe is frequently not emptied in a single process but is stored until it is reused. The material inside is thus discharged over several applications. In this case, it is essential that after a discharge of the material from the discharge opening or discharge nozzle of the syringe, no material leaks out. On one hand, such an uncontrolled discharge would thereby interfere with the discharge of the proper amount, and on the other hand, before the discharge nozzle is closed by attaching the cap, no material should adhere to the surfaces which come into contact with the cap. For the reasons indicated above, the rotary piston is usually connected to the stopper so that the rotary piston, as it is being rotated back opposite to the discharge direction, pulls back the stopper and thus relieves the pressure on the material, or an underpressure is created in the cartridge so that no additional material is! discharged from the discharge nozzle. But with this configuration, when the stopper is in very tight contact against the inside wall of the cartridge, the material is pulled back very far into the cartridge, so that the rotary piston must be twisted back into the cartridge very far to resume the discharge of the material from the cartridge. Furthermore, the rotary piston is practically a permanent component of the stopper so that after such a syringe has been used the rotary piston with the stopper cannot be reused.
An object of the present invention is to provide a syringe for the controlled discharge of pasty materials, such as dental materials, so that when a discharge is finished, the syringe performs a controlled depressurization of the material contained in the cartridge and a good seal of the material is achieved by the stopper over the entire length of the cartridge toward the outside, i.e., toward the side of the rotary piston.